Building a Forest Bridge-Design & Planning

We have a small creek which crosses our road. I am able to get over with both my truck and tractor but most vehicles couldn't get across due to steep banks. The creek runs most of the time but does dry up occasionally (it dried up last summer but not the previous two). If we hope to get any other type of traffic across, we needed to build a bridge. Thinking off into the future, this small bridge will need to be able to support: logging trucks and associated equipment, cement trucks and (hopefully not) a fire truck. A little bit of research defined the heaviest of these to be the logging truck so the bridge will need to be designed to carry in excess of 70,000 lbs (it's been designed for 90).

Our other option was a culvert which we chose not to use for a few reasons. First, it would have required digging up most of the streambed which did not excite me. Second, our state (Maine) dictates that a culvert must go from the streambed on the upstream side to the streambed on the downstream side. For us, this meant a very steep incline as the levels of the stream are different by about three feet. Our little stream lies in the headwaters of the Medomak River Watershed which is one of the few in our area that hasn't had much fish habitat restoration done on it. If we placed a culvert at this steep angle, it might preclude fish migration in the future if there is any chance of it happening at all. Of course this assumes that funds will support barrier removal on this body of water in the future. The third reason was aesthetics. A small bridge is certainly more appealing than a culvert (tho it is a bit more expensive).

When I started planning to build our bridge, cost sharing from our local Forest Service was an option. That was two years ago and those state funds have dried up. Our local state forester directed me to the NRCS (Natural Resources Conservation Service) which is under the larger arm of the USDA (yup, the US Dept of Agriculture). I wasn't real excited about dealing with the federal government and had to sign lots of paperwork. Of course, I had just missed the deadline to get any funding at the beginning of the year and, while congress has approved the most recent Farm Bill this spring, the bureaucratic wheels grind slowly and I wouldn't see any funding till this fall which is when I plan to start harvesting some trees (more on this later on). They were kind enough to arrange for an engineer to visit our stream and he provided me with the necessary size and load details so that I could finish designing the bridge.

Realizing that we'd have to pay for all of this ourselves, I began the process of researching design options as compared to cost options. The easiest and most reasonably priced structure sinks retaining wall blocks halfway into the ground, five steel I-beams sit on top of the blocks spanning the stream and hemlock beams cross atop the I-beams forming a bridge deck. I could have used wooden timbers to cross the stream but these have a more limited life-span and I would have had to rebuild the bridge every 10 years (this is according to NRCS guidelines, realistically, it probably would have lasted much longer than this). My wife has a hard time visualizing things so I used Google SketchUp (sketchup.google.com) to design a 3D model.

As we will be harvesting timber this fall, I filed a Permit By Rule with the State. The details are submitted to our DEP (Dept. Environmental Protection) and only if they find fault with them do they contact you. So, if you don't hear anything within two weeks, you can go and build. I had done my research and provided them with all the details and I had assumed there wouldn't be an issue. They do ask that you send before and after pictures once your project has been completed.

The Ingredients:

Retaining wall blocks will provide the base for the bridge and our local cement company (State Sand and Gravel) makes these at $50 per. These blocks are 2' x 2' x 6' long and weigh in at about 3500 lbs. They were willing to pour in anchor bolts for me so that I can physically secure the I-beams to the blocks. These blocks are a bit heavy for my pickup truck and they do not deliver. So, the good folks at State Sand and Gravel suggested finding a local logging truck with a grapple arm. This way, I could pick up the blocks at the plant, transport them to my bridge site and place them exactly where I needed them to be. Being a state with plenty of logging activity, it was pretty easy to find somebody local willing to do this for me.

On top of the blocks are 5-20' long steel I-beams spaced equidistantly. These are the most expensive part of the bridge at $500 a piece. They are 12" high, weigh about 500lbs each and are easily managed with my tractor. I requested quotes for my steel at three local establishments. I looked at prices for just the steel and for steel + fabrication. Each steel beam needs 26-5/8" holes drilled in the top plate to bolt the hemlock cross beams to as well as a few holes in the bottom plate to bolt the bridge to the cement blocks. The specifications also call for 3 sets of 8" tall x 33" long steel braces welded between each I-beam. One of the companies wouldn't do fabrication but had the best price. The second wanted to charge me over $2000. and their beams were also two hundred more than the first. The third wouldn't get back to me even after I called them twice (nertz to them!). To save two grand, I decided to drill my own holes and do my own welding. I have a neighbor who works for a marine contracting company. He suggested using an annular cutter which cuts plugs in steel instead of boring holes and should be faster. My local rental shop can get a Mag drill (a vertical drill press that attaches to steel with a magnetic base) for $50. for the weekend, the bits cost about $25. each over the internet and I have a generator that will supply electricity for the drill.

We have a local sawmill that mills hemlock. The design calls for 8" x 8" x 14' long beams spaced 1 1/2" apart. I need 26 of these cross beams as well as 2" (x 10") running boards to sit atop the deck. They also carry old rail ties that I will put horizontally in front of the steel I-beams to transition from gravel road to bridge. Mills usually charge by the board foot for their lumber and the wood deck will run about $1200. although (over the phone) they said they'd give me a bit of a break due to the volume.

Hardware, mostly in the form of galvanized 10" carriage bolts will come from my local mom & pop hardware store. Buying in bulk from them was much less expensive than buying from the local big box stores (I checked prices at both). It does pay to shop around.